R. Rodríguez-Clemente

Mechanical properties of visible light-cured resins reinforced with hydroxyapatite for dental restoration.

OBJECTIVES The purpose of this study was to measure and analyze the mechanical properties of several composite materials designed for dental restoration. METHODS The materials were composed of a visible light-curing monomer mixture (either Bis-GMA+TEGDMA or Bis-GMA+HEMA) as a matrix and hydroxyapatite (either microscopic or nanoscopic particles) as a reinforcing filler. The surface of the hydroxyapatite particles was modified by using a coupling agent (citric, malic, acrylic or methacrylic acid). Five specimens of 14 different composites were prepared for each mechanical test: flexural strength, Youngs modulus and Vickers hardness. Mean values and standard deviations were calculated, and ANOVA and Student Newman Keuls multiple comparison tests were applied (P < 0.05). RESULTS The addition of 50-60 wt% of hydroxyapatite particles to the unfilled monomer mixtures led to the increase of both Youngs modulus and surface hardness of the material, while the flexural strength decreased. In general, when microscopic instead of nanoscopic hydroxyapatite was used as a reinforcing filler, mechanical properties were favored. The mechanical properties were also improved by adding citric, acrylic or methacrylic acid as a coupling agent. CONCLUSIONS An adequate surface modification of the hydroxyapatite particles conferred enhanced mechanical properties to the final dental composite. Microscopic-hydroxyapatite particles are preferred to nanoscopic ones.

Nucleation of calcium carbonate at different initial pH conditions

The precipitation of calcium carbonate from mixing calcium chloride and potassium bicarbonate/carbonate solutions at 25°C has been studied doing unseeded batch pH-drift experiments. The record of pH has been used to determine induction times. The pH and pCa measurements were used for calculations of species concentration and supersaturation. SEM indicated that vaterite was the predominant phase formed. The dependence of the induction time on supersaturation was analyzed with different models. The results indicate that a good fit is obtained from the plot of ln ti versus l/ln2β but the small size of the critical nucleus indicates that an atomistic model is more realistic. The nucleation parameters have been determined using an empirical expression of the growth rate of vaterite taken from the literature, together with the experimentally measured induction times.

Precipitation of stoichiometric hydroxyapatite by a continuous method

In this paper we present the precipitation of hydroxyapatite (HA), Ca 5 (OH)(PO 4 ) 3 , from highly concentrated CaCl 2 and K 2 HPO 4 solutions, carried out by a continuous method in a MSMPR reactor. The procedure consists of adding the reagents in a ratio Ca to P equal to 1.67, maintaining a temperature of 85°C, inert N 2 atmosphere inside the reactor, and monitoring and adjusting automatically the pH by means of a pH-stat system (pH =9.0 ± 0.1). Under these conditions HA with a Ca to P ratio equal or close to the stoichiometric composition (Ca/P = 1.667), with a high yield (up to 99%) and a high production rate (up to 1.17 g/l.min) is obtained at steady state. The CSD, morphology, crystallinity of the precipitates and impurities present fit the requirement for its biomedical applications.

Hydroxyapatite precipitation : A case of nucleation-aggregation-agglomeration-growth mechanism

The interpretation of the precipitation data of hydroxyapatite in MSMPR systems presents several problems due to the lack of consistency of the classical nucleation-growth-agglomeration mechanism in the analysis of the Crystal Size Distribution. In the proposed model, the mechanism of formation of particles starts with a nucleation episode, followed by the aggregation of supercritical nuclei ruled by surface energy excess, which counterbalance the repulsive forces between particles. The aggregates agglomerate, forming solid bridges, and the agglomerates continue to grow as distinct particles due to repulsive forces.

Morphological differences in ZnO films deposited by the pyrosol technique: effect of HCl

In this letter, the surface aspect of crystallized ZnO films deposited by the pyrosol technique is examined. Depending on whether the starting solution contains chlorine species or not, the crystal development is either in a pyramidal form or in a prismatic form. The influence of HCl vapor is used to explain these morphological differences.

Morphological properties of chemical vapour deposited AlN films

Abstract In this paper we analyse some results on AIN CVD film deposition, published in the literature, from a morphological point of view, and we propose a model to explain the observed preferential orientations based on the reactivity of the AIN crystal faces of the equilibrium form.

Calcium carbonate precipitation from aqueous solutions containing Aerosol OT

Abstract In supersaturated solutions containing Ca 2+ , HCO − 3 , CO 2− 3 and Aerosol OT (AOT) at 45°C, AOT shows an inhibitory effect in the nucleation of vaterite under certain concentrations (i.e T = ΣC = 0.01M) and blocks its solvent-mediated transformation to calcite at relatively high concentrations, i.e. 200 ppm when [Ca] T = ΣC = 0.01M or 800 ppm when [Ca] T = ΣC = 0.1M. This last phenomenon is explained by the total coverage of the vaterite surface. Most probably, the formation of a double AOT layer has taken place, in a way similar to that observed in the aqueous system AOT-calcite. When supersaturated solutions are seeded with calcite, perhaps a change in the growth mechanism from normal growth (for 0 and 13 ppm AOT) to 2D-nucleation mediated growth (at 25 and 50 ppm AOT) takes place. For supersaturation ratios S S > 3.5 the effect is the opposite, AOT acting as a surface catalyst.

TiO2 thin film deposition from solution using microwave heating

Abstract A new method has been devised for the deposition of TiO2 thin films on conducting glass using a microwave reactor. The substrates are immersed in a diluted homogeneous aqueous solution which was prepared by mixing equal volumes of a fluorine-complexed titanium(IV) solution ([Ti]=3.4×10−2 M) and 6.8×10−2 M boric acid solution. Low microwave power and short deposition time have been used. The TiO2 layers obtained are well-adhered, homogenous, with good specularity and colored by interference of reflected light. Their thickness is in the range of 100–500 nm. SEM experiments denote that films are formed by small crystallites having linear dimensions under 100 nm. Crystal dimensions depend on microwave power and deposition time. The layers show a high degree of crystallinity and the observed crystal phase is anatase. Microwave heating has proved to be an efficient and inexpensive method for solution growth of TiO2 films; it should also be of importance for other materials layers grown from solution.

Synthesis and photocatalytic activity of mesoporous anatase prepared from tetrabutylammonium-titania composites

Abstract The synthesis of new titania-based materials obtained from the hydrolysis of titanium(IV) isopropoxide (TIP) in the presence of tetrabutylammonium hydroxide (TBAOH) was investigated. Hybrid composites constituted by tetrabutylammonium (TBA) cations embedded into a titania matrix were produced in a first stage. Powder X-ray diffraction (XRD) patterns of these composites exhibited broad low-angle reflections, which indicates the existence of some degree of ordering on the scale of mesopores. Porous titania with BET surface areas up to 110 m 2 g −1 and diverse distributions of pores in the range of mesopores were obtained after different treatments devoted to the removal of TBA from these composites. The photocatalytic activity of the pure titania final products was tested by the gas phase oxidation of formaldehyde.

Microwave precipitation of CaCO3 from homogeneous solutions

Abstract The precipitation of calcium carbonate, calcite, from homogeneous complexed solutions formed by Ca-citrate and HCO − 3 CO 2− 3 has been carried out in batch and MSMPR reactors activated by microwave radiation. In the batch experiments, monodisperse calcite crystals with ellipsoidal or peanut-like shapes have been obtained under certain experimental conditions, while in the continuous precipitation experiments the calcite crystals present ellipsoidal shapes. These morphologies are unusual for the phase calcite. In the continuous precipitation experiments, the rate of nucleation observed is higher than the one measured in a conventional MSMPR reactor in equivalent conditions. A possible explanation to this fact lies in the specific effect of microwaves in weakening the CaH 2 O bonding of the inner hydration sphere of calcium, thus favoring its dehydration, which is the rate controlling factor in the precipitation of calcium carbonate.